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multiple general- purpose registers, placing a burden for efficient execution on the assembly- language
programmer (e.g., compiler writer).

Register Indirect Addressing

Just as register addressing is analogous to direct addressing, register indirect addressing is analogous to
indirect addressing. In both cases, the only difference is whether the address field refers to a memory
location or a register. Thus, for register indirect address,

EA = (R)

The advantages and limitations of register indirect addressing are basically the same as for indirect
addressing. In both cases, the address space limitation (limited range of addresses) of the address field is
overcome by having that field refer to a word- length location containing an address. In addition, register
indirect addressing uses one less memory reference than indirect addressing

Displacement Addressing A very powerful mode of addressing combines the capabilities of direct
addressing and register indirect addressing. It is known by a variety of names depending on the context
of its use, but the basic mechanism is the same. We will refer to this as displacement addressing:

EA = A + (R)

Displacement addressing requires that the instruction have two address fields, at least one of which is
explicit. The value contained in one address field (value = A) is used directly. The other address field, or
an implicit reference based on opcode, refers to a register whose contents are added to A to produce the
effective address. We will describe three of the most common uses of displacement addressing:

■ Relative addressing

■ Base- register addressing

■ Indexing relative addressing for relative addressing, also called PC- relative addressing, the implicitly
referenced register is the program counter (PC). That is, the next instruction address is added to the
address field to produce the EA. Typically, the address field is treated as twos complement number for
this operation. Thus, the effective address is a displacement relative to the address of the instruction.
Relative addressing exploits the concept of locality that was discussed in Chapters 4 and 8. If most memory
references are relatively near to the instruction being executed, then the use of relative addressing saves
address bits in the instruction.

base- register addressing for base- register addressing, the interpretation is the following: The referenced
register contains a main memory address, and the address field contains a displacement (usually an
unsigned integer representation) from that address. The register reference may be explicit or implicit.
Base- register addressing also exploits the locality of memory references. In some implementations, a
single segment- base register is employed and is used implicitly. In others, the programmer may choose a
register to hold the base address of a segment, and the instruction must reference it explicitly. In this

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